This invention relates to the art of thermoforming plastics, especially to the art thermoforming polyesters and copolyesters.
It is well known that certain thermoformable plastics can degrade during processing. For example, when subjected to extrusion or injection molding polyethylene terepthalate is subject to hydrolytic and thermal degradation, which can lead to a reduction in impact strength and other physical properties and to thermal oxidation which causes discoloration.
Although it is known that in order to avoid decomposition of pellets of polymers which are to be extruded into thermoformable, thermoplastic sheets, a drying step is conducted before the extrusion. However, after the thermoplastic sheet is extruded, no further drying is recommended by the suppliers of the thermoformable plastic sheets. In this field of art, such extruded plastic sheets, for example extruded PETg sheets, are normally thermoformed without bubble problems. According to Wikipedia, “pre-drying [of polyethylene terethalate] is not necessary. http://en.wikipedia.org/wiki/polyethylene terephthalate, accessed Jan. 10, 2011.
In another article published by Larry Koester, Sheila Nemeth, and Mark Koester of Lavergne Group entitled “Thermoforming & Die Cutting of Recycled/Virgin PET Sheet” it is stated “PET sheet does not need any special drying prior to thermoforming, but should not be exposed to rain or water.” See http://www.burchamintl.com/papers/petpapers/34 gpec03.pdf accessed Jan. 14, 2011.
Depending on the manufacturer, the date the thermoformable sheets were made, possible impurities or exceptions to normal manufacturing processes, we have discovered that customers occasionally encounter bubble problems when extruding copolyester sheets such as PETg sheets. In spite of extensive research to determine the cause of such bubble formation, prior to this invention the solution to this problem has proven elusive.
It is an object of the invention to provide a solution to the bubble problem encountered with certain samples of thermoformable copolyester sheets.
A further object is to provide a process for preventing bubble formation as a result of thermoforming of polyester sheet.